function forbidBackSpace(e) {
	var ev = e || window.event; //获取event对象 
	var obj = ev.target || ev.srcElement; //获取事件源 
	var t = obj.type || obj.getAttribute('type'); //获取事件源类型 
	//获取作为判断条件的事件类型 
	var vReadOnly = obj.readOnly;
	var vDisabled = obj.disabled;
	//处理undefined值情况 
	vReadOnly = (vReadOnly == undefined) ? false : vReadOnly;
	vDisabled = (vDisabled == undefined) ? true : vDisabled;
	//当敲Backspace键时，事件源类型为密码或单行、多行文本的， 
	//并且readOnly属性为true或disabled属性为true的，则退格键失效 
	var flag1 = ev.keyCode == 8
			&& (t == "password" || t == "text" || t == "textarea")
			&& (vReadOnly == true || vDisabled == true);
	//当敲Backspace键时，事件源类型非密码或单行、多行文本的，则退格键失效 
	var flag2 = ev.keyCode == 8 && t != "password" && t != "text"
			&& t != "textarea";
	//判断 
	if (flag2 || flag1)
		return false;
}